The HIV-1 envelope (Env)-specific Th cell response of rhesus monkeys infected with nonpathogenic chimeric simian/human immunodeficiency viruses (SHIV) was assessed at a molecular level by establishing rgp120-specific CD4+ T cell lines. Epitopes recognized by these MHC class II-restricted Th cells were identified within the second (C2), third (C3), and fifth (C5) conserved regions of HIV-1 gp120. The epitope located in C2 was found to be restricted by Mamu-DRB*W201. The restriction element requirements of the C2 and C5 epitope-specific CD4+ Th cells appear to be rather stringent, because these peptide epitopes were not recognized in the presence of other Mamu-DR expressing cell lines. The ability to analyze HIV-1 Env-specific CD4+ T cell responses in SHIV-infected monkeys will enhance the utility of this model for studying AIDS pathogenesis and for assessing novel HIV-1 vaccine